Electrochemical cells are widely in electrical energy storage devices because of their light weight relative to other types of batteries. For high power applications such as electric vehicles, there has been a continuing effort to improve the energy output and useful lifetime in such batteries in order to better suit such applications. Lithium-sulfur batteries have a theoretical capacity of 1675 mAhg−1, nearly one magnitude higher than that of LiFePO4 (theoretical capacity of 176 mAhg−1), and hold great promise for such high power applications. The Li/S system has not been adapted for high power applications because of significant obstacles including the poor electrical conductivity of elemental sulfur and the intrinsic polysulfide shuttle, both of which contribute to capacity fade with cycling. After decades of intensive development, lithium ion batteries are still incapable of meeting the energy density requirements of many emerging applications. The exploration of new electrochemistry and new materials is thus necessary for the creation of high-energy battery systems.
There are a number of important classifications of redox reactions involved in electrochemical energy storage in Lithium ion batteries including “conversion” and “intercalation”. While having fundamentally different reaction pathways, each type of reaction has the same goal, namely the storage of electrons to perform work-on-demand. The reversible conversion reaction is characterized by an alkali metal ion reducing an element or compound that undergoes a crystalline and morphology phase change over the course of oxidation-reduction typically associated with soluble intermediate species (crystalline sulfur reduced to amorphous lithium sulfide). The reversible intercalation reaction, commonly described as Li-ion, is characterized by an alkali metal ion insertion/de-insertion into a crystalline lattice changing the oxidation state of a host lattice transition metal ion (lithium cobalt oxide, Co2+/Co3+).
The advancement of Li/S battery technology has not progressed as rapidly as conventional Li-ion batteries. The primary reasons for this is rooted in the complex nature of the electrochemical conversion reaction: 1) intermediate lithium polysulfides (Li2Sx, 8≥x≥6) are soluble in the electrolyte which diffuse out of the cathode resulting in irreversible capacity loss, 2) these soluble polysulfides create a phenomena known as a redox shuttle where the cell can never reach full charge capacity (cannot achieve elemental sulfur from oxidation of lithium sulfide), 3) electronic insulating nature of sulfur increases charge transfer resistance in the cathode limiting the redox cycle rate capability, 4) extreme volume change from sulfur to lithium sulfide (nearly 400%) pulverizes the cathode over many charge-discharge cycles, 5) low order polysulfides (Li2Sx, 6≥x≥1), can precipitate out of the electrolyte making the oxidation step unfavorable, requiring high over-potentials to drive the oxidation. Combined, the resultant electrochemical cell is sluggish, short lived and far from practical. In addition, the electrical conductivity of elemental sulfur can be as low as 5×103 S/cm at 25° C. Such a low conductivity causes poor kinetics leading to low utilization of active materials in the cathode. Although compositing elemental sulfur with carbon or conducting polymers can improve the electrical conductivity of sulfur-containing cathodes, the porous structure of the cathode still needs optimization to facilitate the transport of ions while retaining the integrity of the cathode after dissolution of sulfur at the discharge cycle.
There is a need in the art for batteries with improved performance, particularly with respect to initial discharge capacities, cycling performance, rate capability, and electrical power output (i.e., improved power density), as well as improved usable lifetime.